Power cable for high-frequency oscillators



Oct 22, 1946. MQLES 2,409,640

POWER CABLE magma FREQUENCY OSCILLATORS I Filed Sept.- 18. 1942' 5 i 50 28 5 I3 55 3 2o 33 2s 4 32 40 COPPER Inventor: Frank J. Moles, by JAM I His Attorney.

Patented Oct. 22, 1946 POWER CABLE FOR HIGH-FREQUENCY OSCILLATORS Frank J. Moles, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application September 18, 1942, Serial No. 458,821

' 4 Claims.

My invention relates to cables and particularly to cables adapted for use in the power supply circuits of short wave electron discharge apparatus, such as oscillators, amplifiers, and the like.

It is an object of my invention to provide an improved cable which has both high attenuation at short wave length and has low distributed capacity.

Another object of my invention is to provide a cable which may be manufactured at a low cost, which is easily constructed from available materials and may be applied to existing high frequency oscillators.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 shows an ultra high frequency oscillator employing the cable of my invention and Fig. 2 shows a preferred embodiment of my high frequency cable.

Referring to the drawing, I have illustrated my improved cable as employed in the power supply circuits of an ultra high frequency oscillator l9 which is shown to be of the resonant cavity type and to which operating potentials are supplied by means of the power leads H|3.

Since the particular form of the ultra high frequency oscillator forms no part of the present invention, its structure and operation will not be described in detail. Briefly, the cavity oscillator i0 employs three telescoped and radially separated conductors i l-J6 which define concentric space-resonant cavities between them. These are used in combination with an electronic tube I! having its control electrode l8 coupled to the intermediate conductor l and its anode l9 and cathode 26 coupled respectively to the remaining conductors. At their upper ends the conductors are secured to a common base 2|, which may be either of metal or of insulating material. By virtue of their'mutual spacing, the conductors provide, in effect, a pair of concentric transmission line sections, each of which by proper termination can be made to function as an oscillatory circuit having a, particular resonant frequency. The movable short-circuiting rings 22 and 23 are provided for adjusting the length of these lines. Annular metal short-circuiting rings 22 and 23, each of which contacts a pair of the conductors l4l6, are provided to adjust the length of the transmission line in question, the handles 24 being provided to adjust the position of the ring 22 and the concentric transmission line 25 serving as a handle to adjust the position of the ring 23. The coupling loop 26, at the end of the transmission line 25, extends into the cavity between the conductors l5 and I6 and provides a means for abstracting high frequency energy from this cavity when it is in excited condition.

The extremities of the conductors l4, l5 and I6 which are remote from the base part 2|, are provided respectively with the finger members 21, 28 and 29 for engagement with the metal disks 3B32, disk 36 being connected to the anode l9, disk 3| to the control electrode l8 and disk 32 to the cathode 20. The insulators 33 of any suitable insulating material, such as glass, provide means for insulatingthe respective elements of the device IT and holding the disks 3ll--32 in spaced relation. The metal fingers 21 are separated from the conductor 16 by a sleeve 34 of any suitable insulating material.

The cathode 29 is indirectly heated by the filament 35, current for filament 35 being supplied through the inner conductors of the cables :2 and i3. Operating potential for the anode I9 is supplied over the inner conductor of the cable I conductively connected to the rod-like member 38 having fingers 31- for engagement with the anode [9. The insulating sleeve 38 is provided to insulate the end member 2| from the high potential present on the rod member 36.

End member 39, engaging the bottom of tube H, is provided to securely hold tube IT in a fixed position in the oscillator and to adjust the pressure of fingers 2'l29 on disk 3B32. Insulator 40 of rubber, or any similar material, prevents lateral displacement of tube H. The cavity resonator and discharge device described above are not my invention and are fully described and claimed in copending Beggs application Serial No. 436,633, filed March 28, l942,,and copending Jensen et al. application Serial No. 448,206, filed June 24, 1942.

In the operation of the oscillator thus far described when the cathode 20 is heated by the filament 35 and high positive potential is sup plied to the anode I9, since the grid and cathode operate at the same potential for unidirectional current, current flows between the anode l9 and the cathode 20. Assuming proper tuning of the inner and outer transmission line sections, that is by proper adjustment of the shorting rings 22 and 23, high frequency oscillations may be developed. The sleeve of insulation 34 serves to isolate the conductor [6 from the high unidirectional potentials present on the anode I9 and.

also serves capacitively to couple conductor l6 and the anode IQ for high frequency voltages. As mentioned previously, the control electrode It and cathode it are operated at the same potential for unidirectional voltages and for high fre-- quency currents at a potential determined by the tuning of the transmission line comprising the conductors M and I5.

In the operation of an oscillator of this type, considerable difficulty has been experienced in confining the high frequency energy to the resonant cavities of the oscillator and the ooncentric transmission line 25 and in preventing this high frequency energy from flowing over the power leads ll-IS. Such leakage of high frequency energy results both in the occurrence of undesirable currents of high frequency in the power supply and any outside measuring circuits and in a reduced output and efficiency of the oscillator.

In order to prevent the leakage of high frequenc energ over the power leads supplying low frequency energy to an oscillator, such as the oscillator in, in accordance with my invention the cable illustrated in Fig, 2 is provided. This cable comprises the inner conductor 55 and the outer conductor or sheath both conductors being preferably of a low resistance material, such as copper. As shown in Fig. 2, the outer conductor 5| is in spaced relation with respect to the inner conductor 5|! and the space between these conductors is filled with a mass of finely divided, or fibrous, conductive material of relatively high resistance, such as steel wool. The high resistance material 52 is insulated from the conductor 50 by the insulating sleeve 53 and from the sheath 5|, by the insulating sleeve 54.

In the use of a cable of the type shown in Fig. 2 with power leads connected to a high frequency oscillator, the inner conductor 50 provides a low resistance path for the power current, while the finely divided metal fibers 52 surrounding the insulating conductor 50 act as a large number of mutually coupled circuits having high resistance to the high frequency currents flowing in them. This material 52 of the cable thus dissipates the high frequency energy which otherwise would escape to outside measuring circuits and the potential source. The insulating sleeves 53 and 54 maintain a relatively low distributed capacity in the cable.

In connecting the cable II to the oscillator Hi, the inner conductor '59 is directly connected to the rod member 35, while the outer sheath 5| is secured to the end member 2| in any suitable manner, such as for example, by soldering. Similarly, in connecting the cables !2 and I3 to the leads of the filament 35 through suitable openings in the end member 39, the outer sheath 5| is secured to the end member 39 While the insulating sleeve 53 serves to insulate conductor 58 from the end member.

In operation it has been found that only a relatively short length of cable is needed to get desired attenuation at very high frequencies, Thus, with an oscillator operating at 3000 megacycles, a cable constructed of a fabric insulated 40-mil wire as the conductor 50, a it; inch layer of steel wool as the finely divided metal 52, an impregnated cambric as the insulating sleeve 54, and a inch copper braid as the outer conductor 5| was observed to have an attenuation of about 3 decibels per inch and a distributed capacity of about 12 micromicrofarads per foot. Thus, a cable constructed as above described is well adapted for attenuating such high frequencies and at the same time keeping a relatively low distributed capacity on the power lead.

It is apparent that various modifications of the above-described cable may be made. Thus, where a flexible cable is not required, a rigid wire may be used as the conductor and a copper tube as the sheath 5!. Also, any finely divided metal, other than steel wool, having a relatively high resistance as compared With the resistance of conductors 50 and 5|, may be used as the filling material 52. In particular, stainless steel has the qualities desire for material 52. Thus, while I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made and I contemplate b the appended claims to cover any such modifications as fall within the true spirit and scope of m invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A filter for removing high frequency oscillations from a power lead comprising a conductive sheath surrounding said lead and spaced therefrom, a mass of metal fibers having a relatively high resistance disposed in the space between said lead and said sheath, and means for insulating said metal from said lead and said sheath whereby power currents of low frequency may be transmitted over said power lead without attenuation by said filter while high frequency currents transmitted over said power lead are highly attenuated.

2. In combination, a low resistance conductor, a low resistance sheath surrounding said conductor and in spaced relation therewith, a continuous layer of finely divided metal of relatively high resistance disposed in the space between said conductor and said sheath, and means insulating said metal from said conductor and said sheath whereby power currents of low frequency may be transmitted over said conductor without attenuation while high frequency currents transmitted over said conductor are highl attenuated.

3. A cable for supplying low frequency currents to a high frequenc oscillator comprising a low resistance conductor, a conductive sheath surrounding said conductor and in spaced rela tion therewith, and a filling of steel wool disposed in the space between said sheath and said conductor, said steel wool being insulated from said sheath and said conductor.

A filter for removing high frequency oscillations from a power lead operating at a potential difference with respect to ground comprising a grounded conductive sheath surrounding said lead and spaced therefrom, a compact mass of fibers of a relatively high resistance metal disposed in the space between said lead and said sheath, and means for insulating said metal fibers from both said lead and said sheath whereby power currents of low frequency may be transmitted over said power load without attenuation by said filter while high frequency currents transmitted over said power lead are highly attenuated.

FRANK J. MOLES. 

